The present invention relates to a flat resistance for a blower control unit of an automobile air conditioner, and particularly relates to a flat resistance having a plurality of resistances connected in series on an electrically insulating substrate for controlling the revolution of the blower. The present invention further relates to such a blower control unit using the flat resistance.
The blower of the automobile air conditioner has a fan installed within a fan scroll. The fan is coupled to a fan motor, of which revolution is controlled by means of the blower control unit. The blower control unit, is provided with a variable resistance circuit including a plurality of resistances connected in series. By varying the resistance of the resistance circuit in a stepwise manner, voltage applied to the fan motor is varied to control the revolution of the fan motor so that the motor is selectively operated in one of a high speed mode, middle speed mode and low speed mode, for example.
In one of typical examples of the conventional blower control unit, each resistance of the resistance circuit is in the shape of a coil and is placed within an air duct of the automobile air conditioner, particularly in the vicinity of a blowout opening of an intake unit thereof, for rapidly removing Joule's heat generated from the resistances by the air stream passing through the air duct. The coil resistances have some resistance to the air stream in the air duct with a rather large pressure loss, and large noises can be produced by the air stream. Thus, the resistances do not meet recent strong requirements of large flow rate of the air conditioner and low noise in the interior of the motor car.
To reduce such drawbacks, a flat resistance in which a plurality of resistances are printed in series on a flat ceramic substrate has been proposed (for example, in Japanese Patent Laid-open (unexamined) Publication 62-88610). The ceramic substrate is inferior in strength and has relatively low heat radiation, which necessitates heat radiating fins. This prior art flat resistance hence requires a considerable thickness, which provides a tendency to cause large pressure loss and noise to be produced when the flow rate of the blower is large.
Accordingly, it is an object of the present invention to provide a flat resistance for a blower control unit of an automobile air conditioner and a blower control unit using the flat resistance, which enables both pressure loss of the forced air and noises produced by the flat resistance to be considerably reduced.